There exists a number of standard illumination models for rendering three-dimensional graphical models with photorealistic results. Such models typically are composed of three-dimensional surfaces defined by geometrical primitives such as surface patches or meshes of polygonal facets. These models may also take into account a number of illuminating light sources.
The main illumination model used in three-dimensional computer graphics is based on the physical characteristics of incident rays of light illuminating a given scene or object. The model generates a variety of illumination components based upon the varying reflective properties of the surfaces being illuminated. These components include:
1. Ambient light, which illuminates all surfaces equally. The intensity of light reflected by a given primitive is dependent only upon the intensity of ambient light and the reflective coefficient of the surface. This results in relatively constant shading across the primitive.
2. Diffuse reflection, in which reflection across the surface varies according to the relationship between the angle to the light source and the surface normal at each point. Intensity increases as the two angles approach each other, and correspondingly decreases as the angles diverge, according to Lambert's Cosine Law. This results in smooth shading across the primitive that includes highlight and shadow regions. Gouraud shading produces this result.
3. Specular reflection, which is one or more sharply focussed highlight regions, typically exhibited by a "glossy" surface. Specular reflection is calculated from the relationship between the angle to the light source, the angle to the view point, and the surface normal. Phong shading produces this result.
The illumination of any point within a three-dimensional scene, relative to a view point, may be determined by adding the red, green and blue reflective contributions for each of the above reflective components, for each light source.
Unfortunately, to provide an accurate representation of a three-dimensional object, it is necessary to calculate intensity for the various colours and components on a per-pixel basis. This, and the accompanying three-dimensional geometry calculations, can place a relatively high load on a computer processor. This in turn limits the speed with which the appearance of a scene may be calculated. For the purposes of interaction, such as for games or multimedia applications, such speed restrictions are undesirable at best.
Accordingly, it is an object of the present invention to provide a method of applying various highlight and/or shadow components to a planar shape to suggest a three-dimensional interpretation thereof without requiring the complex, per-pixel intensity calculations of true three-dimension illumination modelling.